[PATCH] hwrng: core - don't block in add_early_randomness()

[PATCH] hwrng: core - don't block in add_early_randomness()

[Laurent Vivier]

If the device is not ready to provide data the kernel will
be stuck indefinitely in the init function.

This is not a problem if the device is driven using a module,
but if the driver is linked directly into the kernel then the
kernel boot sequence hangs.

This can happen with virtio-rng device with rng-egd backend
with no data provider, for instance with QEMU command line parameters:

...
    -chardev socket,id=charrng0,host=localhost,port=2345,server,nowait \
    -object rng-egd,id=objrng0,chardev=charrng0 \
    -device virtio-rng-pci,rng=objrng0,id=rng0

To avoid that, we can call rng_get_data() in non blocking mode because
the function already manages the case where byte_read is
0 (if the device is not already initialized).

See also commit d3cc7996473a
("hwrng: fetch randomness only after device init")

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
---
 drivers/char/hw_random/core.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/char/hw_random/core.c b/drivers/char/hw_random/core.c
index 95be7228f327..3866d6b8017c 100644
--- a/drivers/char/hw_random/core.c
+++ b/drivers/char/hw_random/core.c
@@ -67,7 +67,7 @@ static void add_early_randomness(struct hwrng *rng)
 	size_t size = min_t(size_t, 16, rng_buffer_size());
 
 	mutex_lock(&reading_mutex);
-	bytes_read = rng_get_data(rng, rng_buffer, size, 1);
+	bytes_read = rng_get_data(rng, rng_buffer, size, 0);
 	mutex_unlock(&reading_mutex);
 	if (bytes_read > 0)
 		add_device_randomness(rng_buffer, bytes_read);
-- 
2.20.1

Re: [PATCH] hwrng: core - don't block in add_early_randomness()

[Herbert Xu]

On Fri, Mar 29, 2019 at 05:30:11PM +0100, Laurent Vivier wrote:
> If the device is not ready to provide data the kernel will
> be stuck indefinitely in the init function.
> 
> This is not a problem if the device is driven using a module,
> but if the driver is linked directly into the kernel then the
> kernel boot sequence hangs.
> 
> This can happen with virtio-rng device with rng-egd backend
> with no data provider, for instance with QEMU command line parameters:
> 
> ...
>     -chardev socket,id=charrng0,host=localhost,port=2345,server,nowait \
>     -object rng-egd,id=objrng0,chardev=charrng0 \
>     -device virtio-rng-pci,rng=objrng0,id=rng0
> 
> To avoid that, we can call rng_get_data() in non blocking mode because
> the function already manages the case where byte_read is
> 0 (if the device is not already initialized).
> 
> See also commit d3cc7996473a
> ("hwrng: fetch randomness only after device init")
> 
> Signed-off-by: Laurent Vivier <lvivier@redhat.com>

I think this is either not a bug (make sure that the user supplies
a RNG source) or it should be addressed in virtio-rng.

Cheers,
-- 
Email: Herbert Xu <herbert@gondor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt

Re: [PATCH] hwrng: core - don't block in add_early_randomness()

[Laurent Vivier]

On 08/04/2019 08:31, Herbert Xu wrote:
> On Fri, Mar 29, 2019 at 05:30:11PM +0100, Laurent Vivier wrote:
>> If the device is not ready to provide data the kernel will
>> be stuck indefinitely in the init function.
>>
>> This is not a problem if the device is driven using a module,
>> but if the driver is linked directly into the kernel then the
>> kernel boot sequence hangs.
>>
>> This can happen with virtio-rng device with rng-egd backend
>> with no data provider, for instance with QEMU command line parameters:
>>
>> ...
>>     -chardev socket,id=charrng0,host=localhost,port=2345,server,nowait \
>>     -object rng-egd,id=objrng0,chardev=charrng0 \
>>     -device virtio-rng-pci,rng=objrng0,id=rng0
>>
>> To avoid that, we can call rng_get_data() in non blocking mode because
>> the function already manages the case where byte_read is
>> 0 (if the device is not already initialized).
>>
>> See also commit d3cc7996473a
>> ("hwrng: fetch randomness only after device init")
>>
>> Signed-off-by: Laurent Vivier <lvivier@redhat.com>
> 
> I think this is either not a bug (make sure that the user supplies
> a RNG source) or it should be addressed in virtio-rng.

Yes, I did more investigation and I agree.

Thanks,
Laurent